1. Field of the Invention
The present invention relates to molded plastic keys, whether individual keys, or plastic card/key combinations on the order of the disclosure in my U.S. Pat. No. 4,677,835 dated July 7, 1987, and is more particularly concerned with reinforcing such keys against breaking at their shanks due to extraordinary twisting stress applied through the handles of the keys.
2. Description of Related Art
Conventional metal keys for operating tumbler locks such as in automobiles, building doors, apparatus controls, and the like, have heretofore been generally constructed throughout the shank and bit portions, and generally the handles, or head portion of a uniform thickness common with the bit thickness. Molded plastic keys have also heretofore been constructed of substantially uniform thickness throughout their length similar to comparable metal keys.
The key bit must be fairly snuggly fitted in the key slot in the cylinder plug. Looseness of the bit in the key slot might defeat proper engagement of the lock tumblers in the key notches. On the other hand, especially in the USA, to avoid difficulty in quick insertion of the key bit into the slot, the general practice has been to provide enlarged entrance dimensions. This results in the key shank often having little if any torque support when subjected to the considerable twisting stress which may occur if for any reason there is resistance to turning of the key in the lock. Such resistance may be variously caused such as by faulty cutting of the key notches, binding due to corrosion or icing, and the like. The general tendency of the user, where there is any such resistance to turning of the key in the lock, is to apply additional torque or twisting force to the key through its handle. Metal keys will generally withstand such extraordinary twisting stress.
In a normal U.S. automobile ignition or trunk lock, only about three to eight inch pounds of torque (IPT) are necessary to open the lock; non U.S. automobiles typically require higher torques. A normal uniform thickness molded plastic key will withstand up to eight to nine IPT. If greater torque is applied there is danger of breaking the plastic key shank. An average woman can apply up to seven to ten IPT and an average man can apply up to ten to fourteen IPT. Therefore, there has been some key shank breakage experienced in respect to molded plastic keys having a substantially common thickness throughout their lengths.
Molded plastic keys have been disclosed in Donald F. Almblad U.S. Pat. No. 4,637,236, and in U.S. Pat. No. 4,677,835 of the present applicant. In both of those patents the keys are shown as of a common thickness throughout their lengths.
By way of a typical disclosure of a metal key and tumbler lock, U.S. Pat. No. 4,656,851 is referred to.